In rechargeable alkaline electrochemical cell, weight and portability are important considerations. It is also advantageous for rechargeable alkaline batteries to have long operating lives without the necessity of periodic maintenance. Rechargeable alkaline electrochemical cells are used in numerous consumer devices such as calculators, portable radios, and cellular phones. They are often configured into a sealed power pack that is designed as an integral part of a specific device. Rechargeable alkaline electrochemical cells can also be configured as larger "cell packs" or "battery packs" that can be used, for example, in industrial, aerospace, and electronics.
Examples of alkaline electrochemical cells are nickel cadmium cells (Ni--Cd) and nickel-metal hydride cells (Ni--MH). Ni--MH cells use a negative electrode having a metal hydride active material capable of the reversible electrochemical storage of hydrogen. Ni--MH cells typically use a positive electrode having nickel hydroxide as the active material. The negative and positive electrodes are spaced apart in an alkaline electrolyte. Upon application of an electrical potential across a Ni--MH cell, the metal hydride material of the negative electrode is charged by the electrochemical absorption of hydrogen and the electrochemical discharge of a hydroxyl ion, as shown in equation (1): ##EQU1## The negative electrode reactions are reversible. Upon discharge, the stored hydrogen is released to form a water molecule and release an electron.
Initially Ovshinsky and his teams focused on metal hydride alloys that form the negative electrode. As a result of their efforts, they were able to greatly increase the reversible hydrogen storage characteristics required for efficient and economical battery applications, and produce batteries capable of high density energy storage, efficient reversibility, high electrical efficiency, efficient bulk hydrogen storage without structural changes or poisoning, long cycle life, and repeated deep discharge. The improved characteristics of these "Ovonic" alloys, as they are now called, results from tailoring the local chemical order and hence the local structural order by the incorporation of selected modifier elements into a host matrix. Disordered metal hydride alloys have a substantially increased density of catalytically active sites and storage sites compared to single or multi-phase crystalline materials. These additional sites are responsible for improved efficiency of electrochemical charging/discharging and an increase in electrical energy storage capacity. The nature and number of storage sites can even be designed independently of the catalytically active sites. More specifically, these alloys are tailored to allow bulk storage of the dissociated hydrogen atoms at bonding strengths within the range of reversibility suitable for use in secondary battery applications.
Some extremely efficient electrochemical hydrogen storage materials were formulated, based on the disordered materials described above. These are the Ti--V--Zr--Ni type active materials such as disclosed in U.S. Pat. No. 4,551,400 ("the '400 Patent") to Sapru, Hong, Fetcenko, and Venkatesan, the disclosure of which is incorporated by reference. These materials reversibly form hydrides in order to store hydrogen. All the materials used in the '400 Patent utilize a generic Ti--V--Ni composition, where at least Ti, V, and Ni are present and may be modified with Cr, Zr, and Al. The materials of the '400 Patent are multiphase materials, which may contain, but are not limited to, one or more phases with C.sub.14 and C.sub.15 type crystal structures.
Other Ti--V--Zr--Ni alloys are also used for rechargeable hydrogen storage negative electrodes. One such family of materials are those described in U.S. Pat. No. 4,728,586 ("the '586 Patent") to Venkatesan, Reichman, and Fetcenko, the disclosure of which is incorporated by reference. The '586 Patent describes a specific sub-class of these Ti--V--Ni--Zr alloys comprising Ti, V, Zr, Ni, and a fifth component, Cr. The '586 Patent, mentions the possibility of additives and modifiers beyond the Ti, V, Zr, Ni, and Cr components of the alloys, and generally discusses specific additives and modifiers, the amounts and interactions of these modifiers, and the particular benefits that could be expected from them.
In contrast to the Ovonic alloys described above, the older alloys were generally considered "ordered" materials that had different chemistry, microstructure, and electrochemical characteristics. The performance of the early ordered materials was poor, but in the early 1980's, as the degree of modification increased (that is as the number and amount of elemental modifiers increased), their performance began to improve significantly. This is due as much to the disorder contributed by the modifiers as it is to their electrical and chemical properties. This evolution of alloys from a specific class of "ordered" materials to the current multicomponent, multiphase "disordered" alloys is shown in the following patents: (i) U.S. Pat. No. 3,874,928; (ii) U.S. Pat. No. 4,214,043; (iii) U.S. Pat. No. 4,107,395; (iv) U.S. Pat. No. 4,107,405; (v) U.S. Pat. No. 4,112,199; (vi) U.S. Pat. No. 4,125,688 (vii) U.S. Pat. No. 4,214,043; (viii) U.S. Pat. No. 4,216,274; (ix) U.S. Pat. No. 4,487,817; (x) U.S. Pat. No. 4,605,603; (xii) U.S. Pat. No. 4,696,873; and (xiii) U.S. Pat. No. 4,699,856. (These references are discussed extensively in U.S. Pat. No. 5,096,667 and this discussion is specifically incorporated by reference). Ni--MH materials are also discussed in detail in U.S. Pat. No. 5,277,999 to Ovshinsky, et al., the contents of which are incorporated by reference.
Nickel hydroxide has been used for many years as an active electrode material for the positive electrode of alkaline electrochemical cells. The reactions that take place at the nickel hydroxide positive electrode of a Ni--MH electrochemical cell are shown in equation (2): ##EQU2##
The positive electrodes are typically pasted nickel electrodes which consist of nickel hydroxide particles in contact with a conductive substrate. The conductive substrate is typically a porous foam comprising nickel or a nickel alloy. A nickel hydroxide positive electrode ideally possesses the attributes of: 1) high discharge capacity; 2) high charge acceptance and utilization; 3) high electrical conductivity; and, 4) long cycle life.
Conventionally, the nickel hydroxide electrode reaction has been considered to be a one electron process involving oxidation of divalent nickel hydroxide to trivalent nickel oxyhydroxide on charge and subsequent discharge of trivalent nickel oxyhydroxide to divalent nickel hydroxide, as shown in equation (2). Recent evidence suggests that quadrivalent nickel is involved in the nickel hydroxide redox reaction; however, full utilization of quadrivalent nickel has never been achieved.
In practice, electrode capacity beyond the one-electron transfer theoretical capacity is not usually observed. One reason for this is incomplete utilization of the active material due to electronic isolation of oxidized material. Because reduced nickel hydroxide material has a high electronic resistance, the reduction of nickel hydroxide adjacent the current collector forms a less conductive surface that interferes with the subsequent reduction of oxidized active material that is farther away.
Ovshinsky and his teams have developed positive electrode materials that have demonstrated reliable transfer of more than one electron per nickel atom. Such materials are described in U.S. Pat. No. 5,344,728, U.S. Pat. No. 5,348,822, U.S. Pat. No. 5,569,563 and U.S. Pat. No. 5,567,549. The disclosures of U.S. Pat. Nos. 5,344,728, 5,348,822, 5,569,563 and 5,567,549 are incorporated by reference herein. Many of these materials involve gamma phase cycling. Nickel hydroxide material that cycles between the beta(II) nickel hydroxide and gamma nickel oxyhydroxide crystalline phases provides for greater electrode capacity.
However, due to the difference in the volumetric densities between beta(II) nickel hydroxide and gamma nickel oxyhyroxide material, there is expansion and contraction of the material during charge and discharge cycling which can sometimes lead to irreversible damage to the positive electrodes. The expansion and contraction can cause the positive electrodes to swell during charging. This can reduce the number of charge/discharge cycles that the electrochemical cell can withstand by causing mechanical failures of the cell.
There is a need for a structurally modified nickel hydroxide material having microstructural and/or macrostructural modifications which can provide for high discharge capacity and increased utilization. There is also need for a nickel hydroxide material which can cycle between the beta(II) and gamma crystalline phases without significant material degradation.